In one embodiment, this invention relates to a process for improving adhesion of polymeric materials to metal substrates including the steps of intergranular etching a surface of the metal substrate and applying to the intergranular etched metal surface an immersion plated metal, and in another embodiment, the invention relates to a foil having a surface with improved adhesion to polymeric materials. The intergranular etching produces a highly irregular surface profile including deep intergranular crevices. The immersion plated metal is applied from an immersion plating solution which includes one or more plating metals selected from tin, silver, bismuth, copper, nickel, lead, zinc, indium, palladium, platinum, gold, cadmium, ruthenium, cobalt, gallium and germanium and mixtures or alloys thereof. The surface obtained by this process provides enhanced adhesion between the metal substrate and polymeric materials applied to the surface of the metal substrate.
Many highly varied processes of chemically or physically treating the surface of a metal substrate, such as copper, to improve the bonding of the metal to a polymeric material, such as epoxy or polyimide, are used in industries such as printed circuit board (PCB) fabrication. PCBs generally comprise non-conducting or dielectric layers such as a fiberglass/epoxy sheets which are clad with a metal conductive layer such as copper or a copper alloy on one or both surfaces. The metal layer of the PCB, before processing, typically is a continuous layer of copper which may be interrupted by a pattern of plated through-holes linking both surfaces of the board. During processing, selected portions of the copper layer are removed to form a raised copper circuit image pattern, i.e., circuitry. Multilayer PCB""s are typically constructed by inter-leaving such circuit-bearing conductive layers with dielectric adhesive layers into a multilayer sandwich which is then bonded together by applying heat and pressure. The dielectric adhesive layer is often a partially cured B-stage resin, referred to as a prepreg. Production of these types of PCB""s is described in xe2x80x9cPrinted Circuits Handbook,xe2x80x9d Fourth Edition, Edited by C. F. Coombs, Jr., McGraw-Hill, 1996, and in xe2x80x9cPrinted Circuit Board Basicsxe2x80x9d, Second Edition, by Michael Flatt, Miller-Freeman, 1992, the teachings relating to PCB manufacture of both are incorporated herein by reference. Since the conductive layer with an untreated surface does not bond well to the prepreg, various surface treatments have been developed to increase the bond strength between the layers of the multilayer PCB sandwich.
Originally such treatments consisted of oxidizing the metal surface with treatments such as alkaline chlorite solutions (e.g., U.S. Pat. Nos. 2,364,993, 2460,896, and 2,460,898). Over time the treatment evolved (e.g., Slominski and Landau, xe2x80x9cAdhesion Promoting Copper Oxide for Plastic on Printed Circuit Boardsxe2x80x9d Plating, June 1982 pp. 96-99) and the oxide became a xe2x80x98reduced oxidexe2x80x99 (e.g., U.S. Pat. No. 4,642,161). By utilization of a reduced oxide, multilayer printed wiring boards were less prone to a problem known as xe2x80x98pink ringxe2x80x99 where oxide is dissolved adjacent to drilled holes during subsequent through hole plating processes. These methods suffer from several drawbacks. The resulting oxide and reduced oxide treatment surfaces are fragile. Thus, processing was typically done by dipping individual parts into solutions. This method of manufacture is not conducive to high volume manufacturing which generally prefers continuous processing such as horizontal conveyorized treatment. Further, due to possible re-oxidation by atmospheric oxygen, reduced oxide treatment requires that layers be bonded (or be otherwise treated or stored to protect from atmospheric oxygen) within a relatively short time (typically less than 48 hours), which also adversely impacts manufacturing time and costs.
Horizontal conveyorized treatment methods eliminating pink ring (Americus C. Vitale, xe2x80x9cDuraBOND Process Eliminates Pink Ring and Wedge Void Defects.xe2x80x9d IPC 32nd Annual Meeting, (April 1989), and allowing extended storage time for the layers (e.g., U.S. Pat. No. 5,073,456) have been developed. Such methods are widely used, employing, e.g., a process sequence comprising immersion tin plating followed by an organosilane coupling agent treatment from, e.g., an aqueous solution. The ""456 patent teaches that the immersion tin coatings alone are insufficient to form a direct bond between the electrically conductive layer (e.g., copper) surface and a dielectric material. The organosilane coupling agent is provided by the ""456 patent as a solution to this problem.
The process of the ""456 patent includes a step referred to as microetching, in which the surface of the metal is briefly treated to form a clean, uniform, microetched surface. In the present specification, the term xe2x80x9cmicroetchedxe2x80x9d includes cleaning and/or pretreating methods in which an amount of metal equivalent to less than 40 microinches, usually less than 20 microinches, is removed from the surface of the treated metal substrate and in which, after the microetching, the surface is quite regular, being substantially free of deep intergranular crevices and steep-sided ridges and valleys.
The method for calculating the amount of metal removed in an etching process is based on the weight loss of a coupon of metal of a given area, which provides an average of the actual etch depth. This method is more fully described below. Such microetching results in a uniform, lightly microetched metal surface which facilitates application of a tin oxide/hydroxide layer by immersion plating, and which is relatively smooth. Such metal surfaces are free of deep, intergranular crevices, although some relatively isolated, angular-sided copper grains may be exposed on the microetched surface.
Recently a xe2x80x98new generationxe2x80x99 of continuous (e.g., horizontal conveyorized) processing methods which use neither immersion tin nor silane treatments to promote adhesion of a metal surface to a polymeric material has been taught (e.g., WO 96/17975, U.S. Pat. No. 5,869,130, WO 99/40764). These methods rely on the resulting roughness and high irregularity of the surface for mechanical enhancement of adhesion between the metal substrate and a polymeric material, such as a prepreg. These methods typically employ a strong acid/oxidizing agent intergranular etching solution, such as a sulfuric acid/hydrogen peroxide solution, modified by the addition of an inhibitor such as benzotriazole (or related compounds) and other additives, such as quaternary ammonium chloride surfactants, sodium chloride, or triphenyl-sulfonium chloride. These methods produce an intergranular etch of the metal surface resulting in a surface morphology characterized by the presence of steep-sided ridges and valleys and/or deep intergranular crevices, and may include a light coating of an oxide over the entire surface. In some cases the solutions include an additional inhibitor such as sodium meta-nitrobenzene sulfonate (U.S. Pat. No. 6,036,758).
Another xe2x80x98new generationxe2x80x99 continuous processing method (U.S. Pat. No. 5,807,493) also produces an intergranular etch, including deep intergranular crevices as described above, but does not result in an oxide uniformly over the surface. This method employs an etching chemistry based upon an organic acid having a pKa of at least 5, such as formic acid, with copper ion and chloride ion, to produce the intergranular etch. The etch step is followed by a desmutting step (e.g. hydrochloric acid xe2x80x98picklingxe2x80x99).
Methods for improving adhesion of metallic substrates to non-conductive, polymeric surfaces of PCBs have included application of additional metal layers by electrodeposition, e.g., Luce et al, U.S. Pat. No. 4,260,449 and Sadey et al, U.S. Pat. No. 6,042,711, and by immersion tin plating, e.g., Holtzman et al, U.S. Pat. No. 4,882,202 and Bokisa, U.S. Pat. No. 5,928,790. The electrodeposition processes provide improved peel strengths, but are time- and energy-intensive. The immersion tin processes have not yielded peel strengths which are significantly better than the above-described intergranular etching processes.
U.S. Pat. No. 4,882,202 describes many prior art processes for improving the bonding strength between a metal substrate and a laminated non-conductive polymeric layer applied thereto. Among the prior art process described is microetching the copper with ammonium persulfate or hydrogen peroxide, although the ""202 patent states that only slightly improved bonding strengths were obtained as a result. The ""202 patent describes an improved immersion tin plating process which includes both a thiourea compound and a urea compound as bath additives. The ""202 patent states that its process allows coating of metal substrates with substantially no pre-cleaning, and that it is not necessary to treat the metal surface (such as by etching) to remove oil or metal oxides. However, the ""202 patent fails to provide any evidence that its process actually improves the peel strengths of PCBs made with its process, as compared to the prior art.
U.S. Pat. No. 5,928,790 describes a process for making multilayer PCBs, including a step of immersion metal plating followed by a step of applying a silane to improve the bond strength between a metal substrate and a laminated non-conductive polymeric layer applied thereto.
The present inventors have discovered that subsequent immersion metal plating treatment of intergranular etched foils with an immersion plated metal provides an unexpectedly significant increase in the peel strengths of such foils and that this invention can be utilized without significantly increasing production costs. The present application discloses and claims the newly discovered methods.
The present invention relates to a process for treating a metal substrate to improve adhesion of polymeric materials thereto, comprising the steps of
intergranular etching a surface of the metal substrate; and
applying an immersion plated metal to the intergranular etched surface by immersing the surface in an immersion plating composition comprising one or more plating metals selected from tin, silver, bismuth, copper, nickel, lead, zinc, indium, palladium, platinum, gold, cadmium, ruthenium, cobalt, gallium and germanium. In one embodiment, the immersion plated metal is tin. In one embodiment, the process further comprises a step of applying a silane over the immersion plated metal from an aqueous solution of a silane. In one embodiment, the process further comprises a step of adhering the immersion metal plated surface to a surface of a polymeric non-conductive material.
In this invention the performance of the xe2x80x98new generationxe2x80x99 of horizontal processing methods is improved by treating the intergranular etched surface of the metal substrate with an immersion plated metal. This treatment process optionally may be followed by treatment with one or more silanes from aqueous solution. As a result, improved adhesion of polymeric materials to the conductive metal foil, e.g., copper foil used in, e.g., PCB laminates, may be attained, compared to foils made by previous processes.